Abstract With a growing number of Earth observation (EO) products available through operational programmes such as the European Union’s Copernicus, there is increasing emphasis on product accuracy and uncertainty, necessitating evaluation against in situ reference measurements. Whilst existing reference datasets have proven a valuable resource, they incorporate little data with which products from recent EO instruments can be assessed. A reliance on individual field campaigns has also led to several inconsistencies, whilst limiting the extent to which temporal variations in EO product performance can be captured. Recently established environmental monitoring networks such as the National Ecological Observatory Network (NEON), which collect routine in situ measurements using standardised instruments and protocols, provide a promising opportunity in this respect. The Copernicus Ground Based Observations for Validation (GBOV) service was initiated in recognition of this fact. In the first component of the project, raw observations from existing networks have been collected and processed to provide reference data for a range of EO land products. In this study, we focus on leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (FAPAR). Raw digital hemispherical photography (DHP) from twenty NEON sites was processed to derive in situ reference measurements, which were then upscaled to provide high spatial resolution reference maps. Using these data, we assess the recently released Copernicus Global Land Service (CGLS) 300 m Version 1 (V1) products derived from PROBA-V, in addition to existing products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Radiometer Suite (VIIRS). When evaluated against reference data, the CGLS 300 m V1 product demonstrated the best agreement (RMSD = 0.57 for LAI and 0.08 for FAPAR), followed by the Collection 6 VNP15A2H and MOD15A2H products (RMSD = 0.81 to 0.89 for LAI and 0.12 for FAPAR). Differing assumptions of the products and in situ reference measurements, which cause them to be sensitive to slightly different quantities, are thought to explain apparent biases over sparse vegetation and forest environments. To ensure their continued utility, future work should focus on updating the GBOV in situ reference measurements, implementing additional corrections, and improving their geographical representativeness.

中文翻译：

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使用哥白尼地面观测验证数据评估北美的全球叶面积指数和吸收的光合有效辐射产物的分数

摘要 随着越来越多的地球观测 (EO) 产品通过诸如欧盟的哥白尼之类的业务计划提供，人们越来越重视产品的准确性和不确定性，因此需要根据原位参考测量进行评估。虽然现有的参考数据集已被证明是一种宝贵的资源，但它们包含的数据很少，可以用来评估最近 EO 仪器的产品。对个别现场活动的依赖也导致了一些不一致，同时限制了可以捕捉到 EO 产品性能的时间变化的程度。最近建立的环境监测网络，例如国家生态观测网络（NEON），它使用标准化仪器和协议收集常规的原位测量值，在这方面提供了一个充满希望的机会。哥白尼地面验证观测 (GBOV) 服务是在认识到这一事实后启动的。在项目的第一部分，收集和处理来自现有网络的原始观测数据，为一系列 EO 土地产品提供参考数据。在这项研究中，我们关注叶面积指数 (LAI) 和吸收的光合有效辐射分数 (FAPAR)。处理来自 20 个 NEON 站点的原始数字半球摄影 (DHP) 以获得原位参考测量值，然后将其放大以提供高空间分辨率参考图。使用这些数据，我们评估了最近发布的源自 PROBA-V 的哥白尼全球土地服务 (CGLS) 300 m 版本 1 (V1) 产品，除了源自中分辨率成像光谱仪 (MODIS) 和可见光红外辐射计套件 (VIIRS) 的现有产品之外。根据参考数据进行评估时，CGLS 300 m V1 产品表现出最佳一致性（LAI 的RMSD = 0.57，FAPAR 的RMSD = 0.08），其次是Collection 6 VNP15A2H 和MOD15A2H 产品（LAI 的RMSD = 0.81 到0.89，FAPAR 的RMSD = 0.12） . 产品的不同假设和原位参考测量导致它们对略有不同的数量敏感，被认为可以解释稀疏植被和森林环境的明显偏差。为确保它们的持续效用，未来的工作应侧重于更新 GBOV 原位参考测量、实施额外的校正并提高其地理代表性。